"These four ingredients paid a huge respect to onion." What an insightful way to interpret this whole new flavor combination. It is an amazing dish on a cold winter night. Thanks for keeping an open mind when trying something that seems so strange! It can really pay off.
French onion soup is not only very healthy and an excellent soup to have in the winter, it's actually very economical to make because you can make the broth out of leftover beef bone, which is cheap and off the charts as far as health benefits goes. Onions are such an inexpensive ingredient and also have a great deal of nutritional value, especially during the winter months. And the traditional French sourdough that would have been used for this recipe is so much more healthy then pasta noodles or dumplings. In the healthy fat and protein from cheese complete the meal. When I'm making beef broth for something like french onion soup I use split marrow bones. Then I reserve the marrow to spread on the toasted bread before I top it with the cheese. It adds another level of richness to this soup along with even more Health benefits. Just make sure if you do this, you get good quality pasture-raised beef bone so it isn't full of toxins.
I make a French onion soup; it's a favorite of my mom. The trick is to not caramelize the onion, but to cook it even lower and slower, to turn it into an onion jam.
Mostly what is available around them during that time of year. Anything out of season is a given to be more expensive but for them, it won't be a thing. Also they follow some arcane practices that are still applicable today.